As communication means in remote areas where network infrastructures are unavailable or at the time of disaster, autonomous distributed (self-configuring) mobile ad-hoc networks (MANET) have come to attention in recent years, in which data is forwarded from one communication terminal to another by wireless communication, and implementation of information sharing in a large-scale environment in which a wide range of many communication terminals (nodes) participate, as well as network infrastructures, is desired.
In order to share information among a wide range of many communication terminals in a mobile ad-hoc network (MANET), especially the following two kinds of information need to be shared among communication terminals in the network:    (1) Node information (i.e. node IDs, node IP addresses, and information about contents held by nodes) and    (2) Routing information (i.e. next-hop (forwarding destination) node information for reaching a destination node, the probability of reaching the destination node, and the routing address of the destination node).
This is because the mobile ad-hoc network uses a mechanism in which each communication terminal identifies an “information sharing terminal” with which the communication terminal is to communicate by “(1) node information” and identifies a “communication (data transfer) route” to the aforementioned “information sharing terminal” by “(2) routing information”.
Accordingly, each communication terminal (node) needs to be in a state where they manage “(1) node information” relating to each of other communication terminals in a network and “(2) routing information” in association with each other or hold both kinds of information.
On the other hand, the amount of information exchanged between communication terminals needs to be reduced because communication bandwidth available between communication terminals in a mobile ad-hoc network (MANET) is limited. In other words, since communication terminals in the mobile ad-hoc network (MANET) directly communicate with each other by wireless communication, interference occurs between radio waves from communication terminals that participate in the network, thus limiting available communication bandwidth. Furthermore, since communication radio waves can fail to reach communication terminals due to movement of the communication terminals or interruption of radio waves by obstacles, the communication link between communication terminals can often disconnected or network disruption can occur.
Consequently, communication bandwidth and time available to one communication terminal for communication with another communication terminal are limited.
In order to enable information sharing in a large-scale environment in mobile ad-hoc networks (MANET) under these circumstances or delay/disruption tolerant networks (DTN) which cope with a poor-quality communication environment in which the network can be disrupted, a method is used in which each communication terminal holds for some time node information and routing information relating to a correspondent communication terminal with which the communication terminal was once connected in order to enable “(1) node information” and “(2) routing information” to be exchange between many communication terminals.
For example, a communication terminal exchanges node information and routing information relating to every other communication terminal that the terminal has met or neighbored and stores the information for a certain period of time in methods for managing information in a delay/disruption tolerant network (DTN) described in NPL 1, “Probabilistic routing in intermittently connected networks” by Anders Lindgren et al. (ACM SIGMOBILE Mobile Computing and Communications Review, Volume 7, Issue 3, July 2003, Pages 19-20 and NPL 2, “ROUTING IN INTERMITTENTLY CONNECTED MOBILE AD-HOC NETWORKS AND DELAY TOLERANT NETWORKS: OVERVIEW AND CHALLENGES” by ZHENSHENG ZHANG et al. (IEEE Communications Surveys & Tutorials, 1ST QUARTER 2006, VOLUME 8, NO. 1, Pages 24-37). Storing node information and routing information as a history for a certain period of time in this way enables a communication terminal to identify a next communication terminal to which data is to be passed when a communication link is recovered or when the communication terminal meets another communication terminal.
However, the amount of information to be exchanged between communication terminals increases in proportion to the number of communication terminals participating in the network. Given these circumstances, in order for communication terminals (nodes) to share information with each other in a mobile ad-hoc network (MANET) in which a wide range of many communication terminals participate or a delay/disruption tolerant network (DTN), which is liable to network disruption, the amount of information exchanged between communication terminals (nodes) needs to be minimized to reduce load on communication bandwidth by cleverly managing and exchanging node information and routing information.
As described above, for information sharing in mobile ad-hoc networks (MANET) or delay/disruption tolerant networks (DTN), “(1) node information” and “(2) routing information” needs to be managed in association with each other or to be held, and the amount of exchanged information relating to both “(1) node information” and “(2) routing information” to be exchanged between communication terminals needs to be reduced.
However, existing network techniques, for example methods proposed in NPL 3 , “Design Issues of Peer-to-Peer Systems for Wireless Ad Hoc Networks” by Dewan Tanvir Ahmed et al. (Networking, 2007. ICN '07. Sixth International Conference on Date of Conference, Pages 22-28, April 2007) and NPL 4, “Scalable routing protocols for mobile ad-hoc networks” by Xiaoyan Hong (Network, IEEE Date of Publication: July/August 2002, Volume: 16, Issue: 4, Pages 11-21) have not achieved a fundamental solution. In the methods, mechanisms for exchanging node information and routing information independently of each other are used and a network is layered and information is managed in a layered manner in order to reduce the amount of information exchanged.
Specifically, in the existing network techniques, since the function of exchanging node information and the function of exchanging routing information operate independently in different network layers and control focusing solely to one of the node information and routing information is performed, the node information and the routing information are not managed or held in pairs. Consequently, a lag between a period in which node information N100 is exchanged and a period in which routing information R100 is exchanged occurs with the other communication terminal (other node) as illustrated in FIG. 11. FIG. 11 is a diagram for illustrating the messaging timings of providing node information and routing information held by a communication terminal (node) and periods during which the communication terminal is holding the node information and routing information in an existing technique. FIG. 11 focuses on a node Y entry relating to node Y received from node X, i.e. a node information entry N100y, and a routing information entry R100y out of the node information N100 held by node A and the routing information R100 held by node A and illustrates a situation where there are differences in the messaging timing of providing information to another node and in period in which the received information is held.
Specifically, with regard to a node information entry N100y relating to node Y that is to be held by node A, for example, there is a period, depicted as node information non-holding period P1 in FIG. 11, between receipt of the routing information entry R100y from node X and receipt of the node information entry N100y from node X during which node A does not hold the node information entry N100y relating to node Y and the routing information entry R100y in pairs because of a difference between the periodicity of exchange of node information N100 and the periodicity of exchange of routing information R100.
Furthermore, because of a lag between a node information N100 holding period and a routing information R100 holding period, the holding period of the routing information entry R100y of node Y, for example, may expire and the routing information entry R100y may be deleted, which may result in a period during which node A holds only the node information entry N100y of node Y out of the two kinds of information relating to node Y that are to be held by node A, as depicted as a routing information non-holding period P2 in FIG. 11.
When node A provides information to the other node at messaging timing T1 and messaging timing T2 in FIG. 11, node A does not provide the routing information entry R100y relating to node Y and provides only the node information entry N100y relating to node Y. As a result, communication bandwidth and storage of the communication terminals are wasted.